Polycarbonate moldings are widely used by virtue of their excellent properties such as transparency, impact strength and tensile strength. The properties of the surface of the material, such as its low resistance to abrasion and scratching and low resistance to solvents, are, however, unsatisfactory for many fields of application. In particular, the surface of the material tends to become opaque in contact with the environment. Numerous methods have been proposed for overcoming this defect, e.g. the use of special coatings based on siloxane or melamine resins. Such processes are described e.g. in U.S. Pat. No. 3,707,397 and U.S. Pat. No. 3,843,390.
These coatings are, however, difficult to prepare and have disadvantages, e.g. these lacquers or coatings are applied from solvents and cross-linking is carried out at elevated temperatures, and this treatment may adversely affect the properties of the polymeric substrate. Moreover, such systems must be built up in several layers and are so brittle that they cannot be used on parts which will be subjected to considerable bending stresses.
It is also known to provide moldings of polycarbonate with a coating a poly(meth)acrylate, e.g. by coating the molded part with a lacquer based on poly(meth)acrylate, as described e.g. in WO 86/04592, or by hot pressing polycarbonate panels with (meth)acrylate films.
Although such a coating on a polycarbonate molding is able to render the system weatherproof, especially when the polyacrylate film applied to the polycarbonate contains a UV absorbent, these coatings have virtually no effect on the scratch resistance.
UV - curable lacquer systems appear to be particularly suitable as coatings systems for polycarbonate owing to their rapid curing. Examples are described in U.S. Pat. No. 3,968,305 and PCT Application WO 80/00968. Neither of these systems, however, is ideal as they both lack sufficient adherence to their substrate and have unsatisfactory mechanical properties and insufficient scratch resistance.
EP 0,274,595 and EPA 0,274,596 describe the use of polyfunctional acrylates as suitable coating compounds. These coatings, however, can only be cured under N.sub.2.
The last two examples given show up a serious disadvantage of radical UV curing, namely O.sub.2 -inhibition. All radically UV curing coating systems suffer from this inhibition to a greater or less extent, i.e. the progress of the reaction is inhibited by atmospheric oxygen. The result is that the surface layers are not cured or not completely cured and the coatings are smeary.
Radically UV cured coatings have the further disadvantage that the reaction cannot be reactivated after the coating has been applied and partly cured; this means that no after-curing takes place. Moreover, the reaction cannot be controlled and proceeds in a completely uncontrolled manner. It would, however, be desirable to control the degree of curing. This would make it possible to coat the moldings with the lacquer, partly cure the lacquer, and only then convert the molding into the required shape and subsequently complete the curing of the lacquer. This would not be possible for brittle acrylate layers as the subsequent shaping process would cause the lacquer to split off or at least form cracks.
Radically UV cured coatings only cure in the areas which are irradiated by UV light. This technology is therefore virtually limited to plane substrates since very spherical substrates, in other words 3-dimensional substrates, contain areas which lie in the "shadow of the light" and therefore cannot be cured. The possibility of reactivation after UV irradiation could solve this problem as these coated 3-dimensional substrates could then first be partly cured by UV irradiation and the coating could subsequently be completely cured by some suitable measure.
It was therefore an object of the present invention to provide irradiation curb coating compositions which when applied to polycarbonates give rise to scratch resistant coatings which are not subject to any inhibition by O.sub.2 and in which the progress of polymerization can be controlled as required by suitable measures.
It has now surprisingly been found that a coating composition based on siloxanes containing epoxy groups and optionally mixed with other compounds containing epoxy groups or hydroxyl groups satisfy these requirements.
UV curable polysiloxanes containing epoxy groups are disclosed in U.S. Pat. No. 4,279,717. They are used for coating paper, polyethylene or polypropylene.
UV curable polysiloxanes containing epoxy groups have also been proposed for release coatings in U.S. Pat. No. 4,576,999.
The curing of these polysiloxanes is promoted by monomeric epoxy compounds (column 10, lines 29 et seq of U.S. Pat. No. 4,576,999).
Suitable photoinitiators are, for example, diethoxyacetophenones and iodonium salts (see Example 19 of U.S. Pat. No. 4,576,999).
Polyester films are mentioned inter alia as suitable for being coated with these materials (column 10, line 62) but the materials are particularly suitable for paper.
It was not obvious in the light of the above to use such systems for coating polycarbonate moldings since in view of the special properties of polycarbonate, e.g. its sensitivity to tension cracking, its chemical resistance, surface properties, etc. one would expect that only coatings specifically adapted go to polycarbonate would be suitable. Further, the compositions mentioned in U.S. Pat. No. 4,576,999 do not have the required high scratch resistance and are relatively soft, flexible lacquer formulations adapted to the mechanical properties of their substrates (paper or plastics film).